1. Field of the Invention
The present invention relates to optical systems. More specifically, the present invention relates to external pupil lens systems.
2. Description of the Related Art
Designing an external pupil imager in the visible spectral band is one of the most difficult tasks in lens design. The difficulty arises from the secondary chromatic aberration correction for the lateral and higher order aberrations such as spherochromatism and chromatic coma. Since the entrance pupil (aperture stop) is external to the optical system, any residual axial chromatic aberration and spherochromatism will introduce a significant amount of lateral chromatic aberration and chromatic coma. The difficulty in correcting the secondary chromatic aberration is due to the nonlinear property of the index of refraction of typical glass materials. Special glass materials such as KzSN4 and PSK52 can be used to minimize the secondary chromatic aberrations, but the lens curvatures will need to be very steep due to the inefficient nature of special glass material in primary chromatic aberration correction. Therefore, the lens tend to be very expensive and are difficult to fabricate and assemble.
The aberrations are more pronounced when the pupil is further away from the lens. This is due to the fact that the intersection of the chief ray on each lens surface is further away from the optical axis. Additionally, the angle of incidence of the chief ray on the lens surface is often very steep. The pupil distance (from the entrance pupil to the first lens) of a typical external pupil lens system is limited to about 0.7 of the effective focal length (EFL). Even with lenses constructed of special glass materials, the entrance pupil distance is still limited to about 1.5 times the EFL.
The entrance pupil distance is determined by the physical mechanics of the system containing the imager. For instance, sensor products for multiple spectral bands commonly have a front end reflective telescope followed by a beam splitting device. The beam splitting device directs different spectral radiation to the corresponding imaging optical systems. To minimize its size, the beam splitting device is located very closely to the telescope exit pupil. The location of this beam splitter dictates the location of the external pupil of the follow up imagers. Therefore, each imager is required to have an external pupil with the pupil distance exceeding 2 to 3 times that of the EFL.
A conventional external pupil lens is therefore a very complex optical system, consisting of many lenses of complex design, some of which are made of special glass materials which are expensive and difficult to fabricate, and having an entrance pupil distance limited to about 1.5 times the EFL.
Hence, a need exists in the art for an improved external pupil lens system with fewer lenses, no need for special glass materials, and a greater allowable entrance pupil distance.
The need in the art is addressed by the present invention, which provides a method for constructing an external pupil lens system with the entrance pupil distance at least three times that of the effective focal length (EFL). The lens system is comprised of several conventional lenses and a diffractive optical element (DOE) for secondary chromatic aberration correction. In the illustrative embodiment, the system includes an entrance pupil, followed by a first lens group containing two refractive elements for primary color correction. Next along the optical axis is a second lens group, which contains two refractive elements for astigmatism and higher order coma correction, followed by a third lens group, which contains one refractive element and one DOE for secondary color correction.
An optical system constructed in accordance with the present teachings is capable of achieving excellent performance even with a pupil distance more than three times the EFL. The number of lenses can be reduced by more than 40% in comparison with a conventional system. More importantly, there is no need to employ any special glass materials or aspheric lenses, which are very expensive and difficult to fabricate. Additionally, the optical power of each lens is much smaller than that of a conventional imager. Therefore, the optical system is less sensitive to alignment error.